VIRTIS-H observations of the dust coma of comet 67P/Churyumov-Gerasimenko: spectral properties and color temperature variability with phase and elevation

Abstract

We analyze 2-5 μm spectroscopic observations of the dust coma of comet 67P/Churyumov-Gerasimenko obtained with the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H) instrument on board Rosetta from 3 June to 29 October 2015 at heliocentric distances rh = 1.24-1.55 AU. The 2-2.5 μm color, bolometric albedo, and color temperature were measured using spectral fitting. Data obtained at α = 90° solar phase angle show an increase in bolometric albedo (0.05-0.14) with increasing altitude (0.5-8 km), accompanied by a possible marginal decrease in color and color temperature. Possible explanations include dark particles on ballistic trajectories in the inner coma and radial changes in particle composition. In the phase angle range 50°-120°, phase reddening is significant (0.031%/100 nm deg-1) for a mean color of 2%/100 nm at α = 90°, which might be related to the roughness of the dust particles. Moreover, a decrease in color temperature with decreasing phase angle is also observed at a rate of 0.3 K deg-1, consistent with the presence of large porous particles, with low thermal inertia, and showing a significant day-to-night temperature contrast. Comparing data acquired at fixed phase angle (α = 90°), a 20% increase in bolometric albedo is observed near perihelion. Heliocentric variations in dust color are not significant in the time period we analyzed. The measured color temperatures vary from 260 to 320 K, and follow a rh-0.6 variation in the rh = 1.24-1.5 AU range, which is close to the expected rh-0.5 value.